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Principle of vacuum precooling
2018/6/20
Principle of vacuum precooling
Vacuum precooling is to place fruits and vegetables in a vacuum chamber with thermal insulation, and vacuum the room with a vacuum pump. When the indoor vacuum reaches the saturation pressure of water vapor corresponding to the temperature of fruits and vegetables, the water in the fiber gap on the surface of fruits and vegetables begins to evaporate, and the evaporation will take away the latent heat of evaporation, which will reduce the temperature of fruits and vegetables, and further reduce the pressure until the fruits and vegetables are cooled to the required temperature
Water boils at 100 ℃ at one atmospheric pressure. However, when the pressure drops, its boiling point will also drop, which can be seen from the boiling point reduction of water on high mountains. At the same time, water absorbs heat for evaporation.
Put vegetables and fruits into a sealed container (vacuum tank), and use a vacuum pump to draw out the air in the tank. The pressure drops. When the original temperature of vegetables and fruits is 30 ℃, and the pressure drops to 32 Torr, the boiling point of water also drops to 30 ℃, the water in vegetables and fruits will boil and evaporate violently. In the vacuum state, the water contained in vegetables and fruits is transported from the center to the surface, while boiling at low temperature. The latent heat of vaporization required in the process of liquid water phase changing into water vapor comes from the sensible heat released by the vegetables themselves, which leads to the rapid and uniform cooling of vegetables and fruits. Further air extraction will make the pressure drop. As the boiling point decreases, the temperature of vegetables and fruits will also drop. When the pressure drops to 6 Torr, the temperature of vegetables and fruits can drop to 0~5 ℃.
Features of vacuum precooling:
(1) Fast cooling speed. It only takes 20~30 minutes for fruits and vegetables to cool from normal temperature to about 0 ℃
(2) Cool evenly. The temperature difference between the center and the surface of fruits and vegetables is very small when cooling
(3) Good quality. The cooled fruits and vegetables have no loss of protein, sugar, lipids, vitamins, etc
(4) Sanitation. Because the precooled fruits and vegetables are in a vacuum environment, which can sterilize or simulate the propagation of bacteria
(5) Low dry consumption. Vacuum precooling can control the evaporation of water in fruits and vegetables by controlling the degree of vacuum, or spray appropriate amount of water on fruits and vegetables before precooling. The dry consumption of fruits and vegetables can be controlled within 2~3%, while the dry consumption of ordinary cooling can be more than 10%
(6) It is not limited by packaging The vacuum cooling speed of fruits and vegetables packed in cartons and plastic bags is no different from that of unpackaged fruits and vegetables
(7) Compared with fruits and vegetables that have not been precooled, it is more suitable for long-distance foreign trade exports and the fresh-keeping treatment of supermarket clean vegetables, pollution-free and pollution-free vegetables, wild vegetables, flowers and edible fungi that are emerging in domestic cities. It has broad prospects.
It has been proved in practical application that the surface of vegetables pre-cooled by vacuum is dry, fresh and green, which is significantly different from vegetables directly cooled by slow blowing in the cold room
Vacuum precooling is especially suitable for vegetables with small body/surface ratio, such as Chinese cabbage, spinach, leek and other leafy vegetables
Vacuum precooling equipment:
The vacuum precooling equipment is specially designed to prevent the decline of freshness and quality of fruits, vegetables, flowers and other agricultural products during storage and transportation. It is a method of placing fruits, vegetables and flowers in a vacuum treatment tank, evaporating water from the surface of fruits, vegetables and flowers under low pressure, and using water to obtain latent heat of evaporation from the surface of fruits, vegetables and flowers to achieve cooling effect.
The vacuum precooling equipment is mainly composed of vacuum chamber, vacuum system, refrigeration system and control system. The function of the refrigeration system is to provide cooling capacity for the water vapor catcher, so that the water vapor evaporated from fruits and vegetables in the vacuum chamber condenses on the cooling coil of the water vapor catcher, and the condensate is discharged without being sucked into the vacuum pump, which affects the pumping performance of the pump
Water boils at 100 ℃ at one atmospheric pressure. However, when the pressure drops, its boiling point will also drop, which can be seen from the boiling point reduction of water on high mountains. At the same time, water absorbs heat for evaporation.
Put vegetables and fruits into a sealed container (vacuum tank), and use a vacuum pump to draw out the air in the tank. The pressure drops. When the original temperature of vegetables and fruits is 30 ℃, and the pressure drops to 32 Torr, the boiling point of water also drops to 30 ℃, the water in vegetables and fruits will boil and evaporate violently. In the vacuum state, the water contained in vegetables and fruits is transported from the center to the surface, while boiling at low temperature. The latent heat of vaporization required in the process of liquid water phase changing into water vapor comes from the sensible heat released by the vegetables themselves, which leads to the rapid and uniform cooling of vegetables and fruits. Further air extraction will make the pressure drop. As the boiling point decreases, the temperature of vegetables and fruits will also drop. When the pressure drops to 6 Torr, the temperature of vegetables and fruits can drop to 0~5 ℃.
Features of vacuum precooling:
(1) Fast cooling speed. It only takes 20~30 minutes for fruits and vegetables to cool from normal temperature to about 0 ℃
(2) Cool evenly. The temperature difference between the center and the surface of fruits and vegetables is very small when cooling
(3) Good quality. The cooled fruits and vegetables have no loss of protein, sugar, lipids, vitamins, etc
(4) Sanitation. Because the precooled fruits and vegetables are in a vacuum environment, which can sterilize or simulate the propagation of bacteria
(5) Low dry consumption. Vacuum precooling can control the evaporation of water in fruits and vegetables by controlling the degree of vacuum, or spray appropriate amount of water on fruits and vegetables before precooling. The dry consumption of fruits and vegetables can be controlled within 2~3%, while the dry consumption of ordinary cooling can be more than 10%
(6) It is not limited by packaging The vacuum cooling speed of fruits and vegetables packed in cartons and plastic bags is no different from that of unpackaged fruits and vegetables
(7) Compared with fruits and vegetables that have not been precooled, it is more suitable for long-distance foreign trade exports and the fresh-keeping treatment of supermarket clean vegetables, pollution-free and pollution-free vegetables, wild vegetables, flowers and edible fungi that are emerging in domestic cities. It has broad prospects.
It has been proved in practical application that the surface of vegetables pre-cooled by vacuum is dry, fresh and green, which is significantly different from vegetables directly cooled by slow blowing in the cold room
Vacuum precooling is especially suitable for vegetables with small body/surface ratio, such as Chinese cabbage, spinach, leek and other leafy vegetables
Vacuum precooling equipment:
The vacuum precooling equipment is specially designed to prevent the decline of freshness and quality of fruits, vegetables, flowers and other agricultural products during storage and transportation. It is a method of placing fruits, vegetables and flowers in a vacuum treatment tank, evaporating water from the surface of fruits, vegetables and flowers under low pressure, and using water to obtain latent heat of evaporation from the surface of fruits, vegetables and flowers to achieve cooling effect.
The vacuum precooling equipment is mainly composed of vacuum chamber, vacuum system, refrigeration system and control system. The function of the refrigeration system is to provide cooling capacity for the water vapor catcher, so that the water vapor evaporated from fruits and vegetables in the vacuum chamber condenses on the cooling coil of the water vapor catcher, and the condensate is discharged without being sucked into the vacuum pump, which affects the pumping performance of the pump
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